For decades, the ability to obtain real time location and position information for mobile platforms and/or individuals has been a highly sough after technology. Since the implementation of the Global Positioning System (GPS), a world-wide radio navigation system introduced by the U.S. Air Force, this has become a reality. The GPS includes a constellation of satellites, ground or base stations, and at least one GPS user receiver.
The locations of the satellites are used as reference points to calculate positions of the GPS user receiver, which is usually accurate to within meters, and sometimes even within centimeters. Each of the satellites, the ground stations, and the GPS user receiver has preprogrammed timed signals that start at precise times. In order to lock on to the signals broadcasted by the satellites, the ground station and GPS user receiver slew their respective internal generated signals relative to time as predicted by their respective internal clocks. When the signals are locked, the GPS user receiver makes ranging measurements to each satellite called pseudoranges. These pseudorange measurements include the actual ranges to the satellites, in addition to an error associated with the receiver clock time offset relative to GPS time, plus other smaller errors. The ground stations included in the GPS control segment network provide ranging measurements which are used to generate predictions for the satellites clocks and orbits. These predictions are periodically uploaded to the satellites and the satellites broadcast this data to the user receiver to support the user receiver positioning function.
Each satellite transmits GPS satellite signals, including a unique Pseudo-Random Noise (PRN) Code and a Navigation (Nav) message, on two carrier frequencies, L1 and L2. The L1 carrier is 1575.42 MHz and carries both the Nav message and the pseudo-random noise code for timing. The L2 carrier is 1227.60 MHz. The L2 signal is normally used for military purposes and is a more precise and complicated pseudo-random noise code. There are two types of PRN codes, called Coarse Acquisition (C/A) code and Precise (P) code. The C/A code, intended for civilian use, modulates the L1 carrier at a rate of 1.023 MHz and repeats every 1023 bits, thus the length of the C/A code is one millisecond. The P code, intended for military use, repeats on a seven-day cycle and modulates both the L1 and L2 carriers at a 10.23 MHz rate. When the P code is encrypted, it is called the “Y” code. Additionally, the Nav message is a low frequency signal added to the codes on L1 and L2 that gives information about the satellites' orbits, their clocks corrections and other system status. Ideally, as the GPS satellite ranging signals are broadcast to Earth, the GPS satellite ranging signals would directly reach the GPS user receiver with a range delay associated with speed of light propagation through a vacuum in an inertial reference system. However, along the route to the GPS user receiver, the GPS satellite ranging signals encounter some sources that cause the GPS satellite signals to be delayed in addition to the path delay associated with the speed of light propagation relative to range prediction models, and thus in error. The potential sources of such delays and errors include satellite ephemeris and clock errors, selective availability (SA), ionospheric and atmospheric effects, multi-paths, and receiver clock error.
In order to reduce or eliminate the delays and errors in the GPS satellite ranging signals, other ground stations, called differential GPS ground stations, are often used. Each differential ground station is stationary and ties all the satellite signal measurements into a local reference. Additionally, a differential ground station closest to the GPS user receiver receives the GPS satellite ranging signals containing the same delays and errors related to the GPS satellite signals as the GPS satellite signals acquired and tracked by the GPS user receiver for the same epoch time. The differential ground station is typically within a few tens of kilometers of the GPS user receiver. The differential ground station measures the range delay or timing errors and then provides this correction information to the GPS user receiver over a radio frequency (RF) wireless communications link. The GPS user receiver may be stationary for the time being, or may be roaming. The GPS user receiver applies these corrections to its ranging measurements to reduce the above errors. The differential ground station knows its fixed position and calculates an expected travel time for each GPS satellite signal. The calculation is based on a broadcast ephemeris of where each satellite should be positioned in space. The differential ground station compares a calculated travel time for the satellite ranging signals to an actual travel time measured for the signals, for all the satellites to determine the error correction information related to the signals for each satellite. The differential ground station then transmits the error correction information for each satellite to the GPS user receiver.
For conventional signal processing, when the GPS user receiver is first turned ON or activated to begin processing GPS signals, it searches for, acquires and locks on to the GPS satellite ranging signals from multiple satellites in view. The GPS user receiver also make distance measurements (called pseudoranges) for each satellite PRN code signal in view of the GPS user receiver, demodulate the Nav message data superimposed on the PRN code signals, apply any error corrections sent to it from the ground station if operating in the differential GPS mode, and uses this information to solve for the GPS user receiver's position and user receiver clock offset relative to GPS time. Additionally, in order to determine the distance between any satellite in view and the GPS user receiver, the GPS user receiver determines the actual travel time for the signal propagation delay and applies the error correction information received from the base station to calculate corrected travel time. The corrected travel time is then multiplied by the speed of light to determine the distance to the signal sending satellite. After acquiring the GPS satellite ranging signals of at least four satellites, the GPS user receiver solves for its position and time error relative to GPS time.
The conventional method of acquiring the GPS satellite ranging signals provides for the GPS user receiver to acquire one GPS satellite signal at a time. Signal acquisition is generally the most fragile phase associated with the GPS user receiver. One reason is that the C/A code is weak and a small level of interference, intermittent attenuation, or obstruction of the Line-of-Sight (LOS) from a satellite to the GPS user receiver can cause the acquisition process to fail for one or more of the ranging signals.
It would therefore be desirable to even further improve the ranging signal acquisition process in a manner that better ensures that rapid, reliable and robust acquisition of the ranging signals can be made by a GPS user receiver in challenged environments associated with signal power attenuation or interference, e.g. in-doors, under foliage, or under jamming conditions.